Ammonia is a widely used chemical with many applications. One specific application is as reductant for selective catalytic reduction (SCR) of NOx in exhaust gas from combustion processes.
For most applications, and in particular in automotive applications, the storage of ammonia in the form of a pressurized liquid in a vessel is too hazardous. Urea is a safe, but an indirect and impractical method for mobile transport of ammonia since it requires urea to be transformed into ammonia by a complex process involving spray, evaporation, thermolysis and hydrolysis ((NH2)2CO+H2O→2 NH3+CO2), which is difficult to achieve under driving conditions with low engine load or cold weather.
A storage method involving ad- or absorption of molecular ammonia in a solid can circumvent the safety hazard of pressurized liquid ammonia and eliminate the problem with conversion of a liquid reductant.
Metal ammine salts are examples of ammonia absorbing and desorbing materials, which can be used as solid storage media for ammonia (see, e.g. WO 2006/012903 A2), which in turn, as mentioned above, may be used as the reductant in selective catalytic reduction to reduce NOx emissions.
When a gas, e.g. ammonia, is generated by thermal desorption from a heated storage unit, it is an advantage for improved safety that the operating pressure of the storage material is not too high above atmospheric level, e.g. 1.5-3 bar (absolute). However, this is an operating range that is the most challenging in terms of low gauge pressure for a gas injector valve and it is also observed that the pressure fluctuates significantly because of the different time scales of the system: fast dosing according to a demand signal (time scale of seconds) and slow response time of pressure as a function of heating (minutes).
Therefore, there is a need for a new method and device for controlled dosing of ammonia from solid ammonia storage systems. Especially for automotive applications, a new method and device should preferably not introduce new, costly equipment—rather use existing types of equipment but embedded in a new configuration and with new control strategy.